1. Field of the Invention
This invention relates to an apparatus for automatically measuring blood pressure such as so-called electronic hematomanometer in which blood pressure is measured by the oscillometric method.
2. Description of the Prior Art
In the oscillometric method, blood pressure is measured on the basis of minute pressure pulses (pulse wave) which are transmitted to a cuff in accompanying with fluctuation of blood pressure of a body to be measured. A conventional hematomanometer employing this oscillometric method measures the maximum and minimum blood pressures as follows: At first, the pressure of a cuff is raised at a single stroke to a high pressure level which exceeds the maximum blood pressure of a subject. Thereafter, the air in the cuff is gradually discharged to reduce the pressure of the cuff. On the way of this discharge, the pulse wave is detected by a pulse pressure sensor to determine the maximum and minimum blood pressure values.
Another hematomanometer which is of a so-called pressure-raising type measures the maximum and minimum blood pressures while increasing the pressure of a cuff. An improved hematomanometer of the pressure-raising type has been proposed in U.S. patent application Ser. No. 199,098 filed May 26, 1988. In the hematomanometer, the pressure of a cuff is raised intermittently as shown in FIG. 9, and the pulse wave is detected at the stopping periods during when the increase of the pressure is halted (i.e., at the pressures P.sub.1, P.sub.2, P.sub.3, . . . ), and the maximum and minimum blood pressure values are determined on the basis of the detected amplitudes of the pulse wave. The U.S. Pat. No. 4,461,266 discloses a hematomanometer employing a detection principle which is similar to that described above.
A hematomanometer of the pressure-raising type has various advantages. For example, as the maximum and minimum blood pressure values are measured while increasing the pressure of a cuff, the time required for compressing the arm of a subject by a cuff of a high pressure can be reduced, thereby eliminating an unnecessary sense of tight binding and pain caused thereby. It is not necessary to set the pressure of a cuff to be raised, in advance of the measurement of blood pressure, resulting in making the hematomanometers easy to operate. As amplitudes of the pulse waves are detected while the pressure-raising operation is halted, moreover, the detected signals are not affected by noises, so that the minimum blood pressure value can be accurately detected even when the minimum blood pressure is low (i.e., the amplitudes of the pulse wave in the vicinity of the minimum blood pressure are low).
However, such a hematomanometer of the pressure-raising type has a disadvantage in that it requires a long period of time for determining the maximum and minimum blood pressure values because the pressure of a cuff is escalated step by step from a zero level.